My research group exploits the opportunities offered by easily accessible and
deep fjord basins for studies of distribution, behavior, mortality and feeding
of zooplankton. Most focus has been on the copepods Calanus finmarchicus
and Paraeuchaeta norvegica, and the krill Meganyctiphanes norvegica.
Current projects on copepods aim at unveiling state-dependent variations in
vertical distribution of P. norvegica, and assessing their feeding using
molecular methods. Projects on krill aim at characterizing swimming behavior
in relation to time of day and feeding mode, and evaluate implications for
encounters
with predators.

To unveil the biology of zooplankton, it is necessary to know their predators,
and studies of distribution, feeding and swimming behavior of planktivorous
fish represent a central part of the research activities. Current projects address
Norway pout (Trisopterus esmarki), whiting (Merlangius merlangus),
herring (Clupea harengus) and sprat (Sprattus sprattus).

We apply echosounders in concurrent studies of fish and plankton. To illustrate,
the echogram below displays the diel vertical migration of macroplankton and
fish at a 120 m deep site in the Oslofjord. The blue layer located at ~70-90
m at day mainly consists of the krill Meganyctiphanes norvegica, the
fish below are Norway pout while the red layers above the krill (most prominent
the second day) consist of schooling clupeids.

Echogram
displaying a diel cycle at a 120 m deep station, and a selected section outlining
data in greater detail, showing the 3-d swimming pattern of a single fish
(identified as Norway pout) at about 100 m depth. Color scales display strength
of the echo (TS; left), which provides information on size, and swimming speed
(right)

Split-beam echo sounders hold the opportunity of defining
the position of an organism in the acoustic beam, and by applying software
allocating subsequent
echoes to the same target, so-called “target tracking” provides data on size,
3-D swimming trajectories and swimming speed of individuals. Target tracking
can normally only be done with the echosounder (transducer) located relatively
close to the targets. Therefore, submerged systems are used to address deep-living
fish. A bottom-mounted, upward looking transducer was used to obtain the results
presented in the lower half of the figure above.

Submersible systems are also used for in situ target
tracking of plankton in deep water, as exemplified below for plankton occurring
at ~80 m depth in the Oslofjord. When lowering the transducer into what appears
as a diffuse echo-layer by a hull mounted transducer, single individuals become
resolved, and individual plankton can be studied in their natural habitat.